Highly porous activated carbon with controlled oxygen content

ABSTRACT

Nanoporous activated carbon material having a high specific capacitance in EDLCs and controlled oxygen content, and methods for making such activated carbon material. Reduction of oxygen content is achieved by (a) curing a carbon precursor/additive mixture in an inert or reducing environment, and/or (b) refining (heating) activated carbon material after synthesis in an inert or reducing environment. The inert or reducing environment used for curing or refining is preferably substantially free of oxygen.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. patent application Ser. No. 12/264,568filed on Nov. 4, 2008, the content of which is relied upon andincorporated herein by reference in its entirety, and the benefit ofpriority under 35 U.S.C. §120 is hereby claimed.

BACKGROUND

The present invention relates to porous activated carbon materialshaving controlled oxygen content and more specifically to highly porousactivated carbon materials and methods for forming highly porousactivated carbon materials having an oxygen content that is lower thanthat for carbon materials produced by conventional processes. Theinvention also relates to high power density energy storage devicescomprising controlled oxygen content carbon-based electrodes.

Energy storage devices such as electric—also calledelectrochemical—double layer capacitors (EDLCs), a.k.a. supercapacitorsor ultracapacitors may be used in many applications where a discretepower pulse is required. Such applications range from cell phones toelectric/hybrid vehicles. An important characteristic of anultracapacitor is the energy density that it can provide. The energydensity of the device, which comprises one or more carbon electrode(s)separated by a porous separator and/or an organic or inorganicelectrolyte, is largely determined by the properties of the carbonelectrodes and, thus, by the properties of the carbon material used toform the electrodes.

Indeed, the performance of an energy storage device comprisingcarbon-based electrodes is largely determined by the physical andchemical properties of the carbon. Physical properties include surfacearea, pore size and pore size distribution, and pore structure, whichincludes such features as pore shape and interconnectivity. Chemicalproperties refer particularly to surface chemistry, which relates to thetype and degree of surface functionalization.

Carbon electrodes suitable for incorporation into EDLCs are known. Highperformance carbon materials, which form the basis of such electrodes,can be made from natural and/or synthetic carbon precursors. Forexample, activated carbon can be made by heating a synthetic carbonprecursor in an inert environment to a temperature sufficient tocarbonize the precursor. During or following the process ofcarbonization, the carbon material can be activated. Activation cancomprise physical activation or chemical activation.

Physical activation is performed by exposing the carbon material tosteam or carbon dioxide (CO₂) at elevated temperatures, typically about800-1000° C. Activation can also be carried out by using an activatingagent other than steam or CO₂. Chemical activating agents such asphosphoric acid (H₃PO₄) or zinc chloride (ZnCl₂) can be combined withthe carbon material and then heated to a temperature ranging from about500-900° C.

As an alternative to performing chemical activation post-carbonization,one or more chemical activating agents can be combined with a carbonprecursor in a curing step prior to carbonization. In this context,curing typically comprises mixing a carbon precursor with a solution ofan activating agent and then heating the mixture in air. In addition tophosphoric acid and zinc chloride, chemical activating agents may alsoinclude KOH, K₂CO₃, KCl, NaOH, Na₂CO₃, NaCl, AlCl₃, MgCl₂ and/or P₂O₅,etc.

In embodiments where a chemical activating agent is used, it ispreferred to homogeneously distribute the chemical activating agentthroughout the carbon precursor at a molecular level prior to curing thecarbon precursor. This molecular level mixing prior to curing enables ahomogeneous distribution of porosity after activating.

The activated carbon product can be washed in an acid or base solutionand then with water to remove both the activating agent and any chemicalspecies derived from reactions involving the activating agent. Theactivated carbon can then be dried and optionally ground to producematerial comprising a homogeneous distribution of nanoscale pores.Activated carbon produced by this method offers significantly higherenergy storage capacity in EDLCs compared to major commercial carbons.

Whether the carbon material is activated using physical or chemicalactivation, the incorporation of oxygen into the carbon, especially inthe form of oxygen-containing surface functionalities, can adverselyaffect the properties of energy storage devices that comprise electrodesmade from the activated carbon. For example, the presence ofoxygen-containing surface functionalities can give rise topseudocapacitance, increase the self-discharge or leakage rate, causedecomposition of the electrolyte, and/or cause a long term increase inresistance and deterioration of capacitance.

Oxygen functionalities can be introduced both during the curing step,where the mixture of carbon precursor and activating agent is oxidizedat intermediate temperatures, and during the carbonization andactivation steps, where the activating agent (e.g., steam or KOH) servesas an oxidation agent.

As a result of the potentially deleterious effects of incorporatedoxygen, it can be advantageous to control and preferably minimize theoxygen content in activated carbon for use in energy storage devicessuch as EDLCs. Accordingly, it would be an advantage to provide a highlyporous activated carbon material having a controlled oxygen content thatcan be used to form carbon-based electrodes that enable high energydensity devices.

SUMMARY

These and other aspects and advantages of the invention can beaccomplished according to one embodiment wherein, after synthesis of anactivated carbon material, a post-synthesis refining step comprisesheating the activated carbon material in an inert or reducingenvironment at an elevated temperature. The inert or reducingenvironment is preferably substantially free of oxygen. According to afurther embodiment, highly porous activated carbon having controlledoxygen content can be prepared by using a modified curing step. In themodified curing step, prior to the steps of carbonization andactivation, a carbon precursor/chemical activating agent mixture isheated to an intermediate temperature in an environment that issubstantially free of oxygen. The substantially oxygen-free environmentcan comprise one or more inert or reducing gases. A still furtherembodiment combines both approaches.

The activated carbon materials produced according to the presentinvention can be characterized by nanoporosity, high specificcapacitance when incorporated into EDLCs, and controlled oxygen content.In one embodiment, the inventive activated carbon has a total oxygencontent of less than 10 wt. %.

Additional features and advantages of the invention will be set forth inthe detailed description which follows, and in part will be readilyapparent to those skilled in the art from that description or recognizedby practicing the invention as described herein, including the detaileddescription which follows, as well as the appended claims.

It is to be understood that both the foregoing general description andthe following detailed description present embodiments of the invention,and are intended to provide an overview or framework for understandingthe nature and character of the invention as it is claimed.

DETAILED DESCRIPTION

The invention relates generally to highly porous activated carbonmaterials and methods for producing such carbon materials. Suchmaterials are especially suited for incorporation into EDLCs. Theactivated carbon materials are, in some embodiments, characterized bynanoporosity, high specific capacitance in EDLCs, and a total oxygencontent that is limited to at most 10 wt. %. By total oxygen content ismeant the sum of all atomic and molecular oxygen in the carbon,including oxygen in oxygen-containing functional groups in and/or on thecarbon.

High nanoporosity and high specific capacitance are valuable for makinghigh performance EDLCs devices, and controlled oxygen content isvaluable for improving the long term stability of the devices, reducingpseudo capacitance, reducing leakage, and reducing electrolytedecomposition. A lower oxygen content also reduces the tendency for thenanoporous carbon material to adsorb water from surrounding environmentand makes it easier to remove water from the carbon material by heattreatment or other means. This is advantageous for manufacturing carbonelectrodes for EDLCs since the amount of water in such electrodes shouldbe minimal

In one embodiment, activated carbon material is subjected to a refiningstep wherein the activated carbon is heated in an inert or reducingenvironment to a temperature ranging from, for example, about 450-1000°C., and for a period of, for example, about 0.5-10 hours. Preferably,the environment during the refining step is substantially free ofoxygen. The refining step advantageously reduces the oxygen content inthe activated carbon. As used herein, when referring to a range ofvalues, the modifier “about” refers to both values in the range. Thus,by way of a clarifying example, disclosure of a temperature in the rangeof about 450-1000° C. means a temperature in the range of about 450° C.to about 1000° C.

In a further embodiment, a mixture of a carbon precursor and a chemicalactivating agent is heated in a substantially oxygen-free environment soas to cure the carbon precursor. This modified curing stepadvantageously reduces the oxygen content in the resulting activatedcarbon, which is achieved by carbonization and activation of the curedcarbon precursor.

In a still further embodiment, a method of forming a porous activatedcarbon material having controlled oxygen content comprises forming amixture of a carbon precursor and an inorganic compound, heating themixture at a first temperature to cure the carbon precursor, and heatingthe cured carbon precursor at a second temperature higher than the firsttemperature to carbonize the carbon precursor and produce an activatedcarbon material, such that the method further comprises at least one of(a) heating the mixture at the first temperature in an environment thatis substantially free of oxygen, and (b) heating the activated carbonmaterial at a third temperature in an environment that is substantiallyfree of oxygen to refine the activated carbon material.

By curing is meant a heating step that at least partially cross-links orpolymerizes a carbon precursor to form a solid material. The curedcarbon precursor can then be carbonized and activated. During thecarbonizing step, the carbon precursor is reduced or otherwise convertedto elemental carbon. During the activation step, which may be conductedconcurrently with the carbonization step, the elemental carbon materialis processed to increase its porosity and/or internal surface area.

In some embodiments, the activated carbon material produced according tothe present invention comprises a total oxygen content of less than 10wt. %. In additional embodiments, the total oxygen content is less than9, 8, 7, 6, 5, 4, 3, 2 or 1 wt. %. The following examples will furtherclarify the invention.

Examples

According to one method for making highly porous activated carbon, anaqueous solution of KOH (45 wt. %) and an aqueous phenolic resin(Georgia Pacific GP® 510D34) are mixed in a ratio of 3:1 by weight. Themixture is cured by heating in an oven at 125° C. for 24 hours and thenat 175° C. for 24 hours to obtain a sponge-like solid with a dull tobrown-yellow color. The atmosphere in the oven is ambient air.

Following curing, the cured resin is broken into small pieces bymechanical force. A known amount (e.g., 250 grams) is placed in agraphite crucible and loaded in a retort furnace (CM Furnaces, Model1216FL) for carbonization/activation. The furnace temperature isincreased at a rate of 200° C./hr to 800° C., held constant at 800° C.for 2 hours, and then cooled down naturally. Throughout the heatingcycle, the furnace is purged with N₂.

Once the furnace temperature has dropped to ambient temperature, the N₂purge is saturated with water vapor by bubbling the N₂ through hotdeionized (DI) water. This step of introducing water-saturated N₂ to thefurnace interior allows any metallic potassium that has been producedduring the heating cycle to react with water vapor and form KOH. Withoutthis step, metallic potassium could self-ignite and possibly explodewhen exposed to oxygen.

The N₂/water vapor purge is continued for 3 hours before the furnace isopened and unloaded. The resulting activated carbon material is soakedin 1 liter of DI water for a few minutes and filtered. It is then soakedin 500 ml of 37% HCl solution for an hour and filtered again. Finally,it is washed repeatedly with DI water until the pH of the effluent isthe same as that of the DI water. These washing steps effectively removeKOH and other derived potassium compounds from the activated carbon.Finally, the activated carbon is dried overnight at 110° C. in a vacuumoven and ground to the desired particle size (typically severalmicrometers).

One typical carbon sample made by the above method was measured usingnitrogen adsorption on a Micromeritics ASAP 2420. The BET surface areawas 1894 m²/g and the total and micro pore volumes were 0.78 cm³/g and0.67 cm³/g, respectively. Carbon made as such typically contains about10 wt. % oxygen by elemental analysis. As mentioned previously, oxygencan be introduced during one or both of the curing andcarbonization/activation steps.

One method to reduce oxygen content is to refine (heat) the activatedcarbon material in an inert environment (such as nitrogen, helium,argon, etc) or in a reducing environment (such as hydrogen, forming gas,carbon monoxide, etc.). A batch of activated carbon material, which willbe referred to hereafter as “standard activated carbon,” was obtained bythe process described above. In a series of experiments, the standardactivated carbon material was divided into several samples that wereheat-treated in a controlled environment at different temperatures.

These refining experiments were conducted in a retort furnace (CMFurnaces, Model 1212FL) purged with nitrogen. To conduct the refining,the furnace temperature was increased at a rate of 200° C./hr. to thedesired refining heat treatment temperature, held constant for 2 hours,and then cooled down to room temperature before exposure to ambientatmosphere.

The standard activated carbon (sample #1) and refined activated carbonmaterial that was heated to 500° C. or 800° C. (samples #2 and #3,respectively) were analyzed for elemental composition and for EDLCperformance in button cells. In these tests, a 1.5M solution oftetraethylammonium tetrafluoroborate in acetonitrile was used as theelectrolyte and the button cells were charged to 2.7V. These analyticalresults are summarized Table 1. In the header of Table 1, theabbreviation “SC” stands for specific capacitance.

It can be seen that the oxygen content was reduced by the refiningtreatment and the magnitude of oxygen reduction was increased byincreasing the refining temperature. The specific capacitance forsamples 2 and 3 was essentially the same as for the standard activatedcarbon on both a gravimetric and volumetric basis. One typical carbonsample refined by heat treatment in N₂ at 800° C. for 2 hours wasmeasured using nitrogen adsorption on a Micromeritics ASAP 2420. The BETsurface area was 1826 m²/g and the total and micro pore volumes were0.75 cm³/g and 0.65 cm³/g, respectively.

TABLE 1 Analytical results for different carbon materials OxygenVolumetric Sam- Content Gravimetric SC ple Process Description [wt. %]SC [F/g] [F/cc] 1 Standard activated carbon 9.5 171 104 (air cure,standard carbonization/activation, no refining heat treatment)(comparative) 2 Refined activated carbon 7.0 170 102 after refining heattreatment in N₂ at 500° C. for 2 hours 3 Refined activated carbon 3.9163 100 after refining heat treatment in N₂ at 800° C. for 2 hours 4Nitrogen (N₂) cure, 8.0 199 99 standard carbonization/ activation, norefining heat treatment

A further method to control the oxygen content in carbon is to cure thephenolic resin/activating agent mixture in an oxygen-free environmentinstead of air. In one experiment, an aqueous solution of KOH (45 wt. %)and an aqueous phenolic resin (Georgia Pacific GP® 510D34) are mixed ina ratio of 3:1 by weight. The resin/activating agent mixture is thencured by heating in a retort furnace (CM Furnaces, Model 1212FL) at 125°C. for 24 hours and then at 175° C. for 24 hours during which time thefurnace is purged with nitrogen gas.

The cured resin, which comprises a sponge-like solid having abright-yellow color, is carbonized and activated using the processdescribed above. In comparison to the standard carbon (sample #1), thiscarbon material (sample #4) shows a substantially lower oxygen content,higher gravimetric specific capacitance, and a comparable volumetricspecific capacitance.

The methods for controlling oxygen content in activated carbon disclosedabove may be practiced over a range of possible variations, including avariety of different materials and processes. The carbon precursor, forexample, can comprise one or more of a natural or synthetic precursor.Examples of suitable natural precursors include coals, nut shells,woods, and biomass. Examples of suitable synthetic precursors includepolymers such as phenolic resin, poly(vinyl alcohol) (PVA),polyacrylonitrile (PAN), etc.

A variety of different activating agents can be used. In addition tosteam and CO₂, suitable chemical activating agents can comprise one ormore inorganic compounds such as KOH, K₂CO₃, NaOH, Na₂CO₃, P₂O₅, AlCl₃,MgCl₂, ZnCl₂ and H₃PO₄. Chemical activating agents can be combined witha carbon precursor in the form of an aqueous solution or, alternatively,in solid form.

During a step of curing with a chemical activating agent, the carbonprecursor and the chemical activating agent can be in the physical formof solid, solid powder, or solution before they are combined. If asolution is used, it is preferably an aqueous solution and theconcentration can range from about 10-90 wt. %. The carbon precursor andchemical activating agent can be combined in any suitable ratio. A ratioof carbon precursor to chemical activating agent on the basis of drymaterial weight can range from about 1:10 to 10:1. For example, theratio can be about 1:1, 1:2, 1:3, 1:4, 1:5, 5:1, 4:1, 3:1, 2:1 or 1:1.

The curing step comprises heating a carbon precursor/activating agentmixture to a temperature in the range of about 100-300° C. for a periodof about 1-48 hours. During the heat-up, hold, and cool-down cycles, themixture is preferably maintained in a reducing or inert environment. Oneor more reducing gases (e.g., H₂, H₂/N₂ mixtures, CO) and/or one or moreinert gases (e.g., N₂, He, Ar) can be used. Further, the environmentduring the curing step is preferably substantially free of oxygen. Asdefined herein, substantially free of oxygen means that the gas phaseoxygen (O₂) content is less than 10 ppm, preferably less than 5 ppm,more preferably less than 2 or 1 ppm.

The carbonization and activation steps comprise heating cured or uncuredcarbon precursor material to a temperature in the range of about650-900° C. for a period of about 0.5-10 hours. As noted above,carbonization and activation can be performed during the same heatingcycle. Alternatively, separate heating cycles can be used to controlcarbonization and activation separately. The heating and cooling ratesfor the carbonization and/or activation steps can range from about10-600° C./hr. As with the curing step, during carbonization andactivation, the environment is controlled to be an inert or reducingenvironment. A preferred environment during both carbonization andactivation is substantially free of oxygen.

The refining step may comprise heating activated carbon to a temperaturein the range of about 450-1000° C. for a period of about 0.5-10 hours.As with the carbonization and activation steps, the heating and coolingrates for the refining step can vary from about 10-600° C./hr, and theenvironment during refining is preferably controlled to be an inert orreducing environment, more preferably an environment that issubstantially free of oxygen. Thus, the refining step maybe conducted ata temperature that is the same as the temperature used in thecarbonization and activation steps, or the refining step may beconducted at a temperature that is greater than or less than thecarbonization/activation temperature.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present inventionwithout departing from the spirit and scope of the invention. Sincemodifications combinations, sub-combinations and variations of thedisclosed embodiments incorporating the spirit and substance of theinvention may occur to persons skilled in the art, the invention shouldbe construed to include everything within the scope of the appendedclaims and their equivalents.

What is claimed is:
 1. A porous activated carbon material formed according to a method, comprising: forming a mixture of a carbon precursor selected from the group consisting of a synthetic polymer thermosetting resin and a synthetic polymer thermoplastic resin and an inorganic compound selected from the group consisting of KOH, K₂CO₃, KCl, NaOH, Na₂CO₃, NaCl, P₂O₅, AlCl₃, MgCl₂, ZnCl₂ and H₃PO₄; heating the mixture at a first temperature to cure the carbon precursor; heating the cured carbon precursor at a second temperature higher than the first temperature to carbonize the carbon precursor and produce an activated carbon material, wherein the method further comprises: (a) heating the mixture at the first temperature in a nitrogen environment, and (b) washing the activated carbon to remove the inorganic compound and then heating the washed activated carbon material at a third temperature ranging from about 450° C. to 1000° C. in an environment that is substantially free of oxygen to refine the activated carbon material.
 2. The porous activated carbon material according to claim 1, consisting essentially of nanoscale porosity and comprising less than 10 wt. % oxygen.
 3. The porous activated carbon material according to claim 1, comprising an oxygen content of less than 8 wt. %.
 4. A high power density energy storage device comprising at least one carbon electrode, wherein the at least one carbon electrode comprises the porous activated carbon material according to claim
 1. 